Ensure that all migration requests are cancellable

[zeripath]

When doing any requests in migration we should set the context for these requests to ensure that they are cancellable.

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-commenter]

Codecov Report

Merging #12669 into master will decrease coverage by 0.00%.
The diff coverage is 12.12%.

@@            Coverage Diff             @@
##           master   #12669      +/-   ##
==========================================
- Coverage   43.30%   43.29%   -0.01%     
==========================================
  Files         646      646              
  Lines       71593    71625      +32     
==========================================
+ Hits        31004    31011       +7     
- Misses      35575    35604      +29     
+ Partials     5014     5010       -4     

Impacted Files	Coverage Δ
modules/migrations/gitlab.go	1.13% <0.00%> (-0.02%)	⬇️
modules/migrations/base/downloader.go	17.64% <5.12%> (-5.72%)	⬇️
modules/migrations/github.go	83.36% <100.00%> (-0.41%)	⬇️
modules/migrations/migrate.go	23.95% <100.00%> (-0.46%)	⬇️
modules/util/timer.go	42.85% <0.00%> (-42.86%)	⬇️
modules/indexer/stats/db.go	43.47% <0.00%> (-8.70%)	⬇️
modules/migrations/git.go	43.33% <0.00%> (-3.34%)	⬇️
modules/git/utils.go	73.77% <0.00%> (-3.28%)	⬇️
modules/log/event.go	57.54% <0.00%> (-1.89%)	⬇️
modules/git/repo.go	49.23% <0.00%> (-0.51%)	⬇️
... and 10 more

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